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| Home > Publications database > Large-scale Investigations of Non-trivial Magnetic Textures in Chiral Magnets with Density Functional Theory |
| Book/Dissertation / PhD Thesis | FZJ-2019-02271 |
2019
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-394-5
Please use a persistent id in citations: http://hdl.handle.net/2128/22015
Abstract: The large-scale Density Functional Theory (DFT) code KKRnano allows one to perform $\textit{ab initio}$ simulations for thousands of atoms. In this thesis an extension of KKRnano is presented and utilized which facilitates the investigation of exotic non-collinear magnetic textures in bulk materials on huge length scales. Such an undertakinginevitably involves the utilization of High Performance Computing (HPC) which is itself a scientific field. The work in this context includes the adaptation of new coding paradigms and the optimization of codes on constantly changing hardware architectures. In KKRnano, the runtime of a simulation scales linearly with the number of atoms due to an advanced Korringa-Kohn-Rostoker (KKR) scheme that is applied, in which the sparsity of the matrices in the multiple-scattering equations is exploited. This enables us to investigate phenomena that occur on a length scale of nanometers involving thousands of atoms.The main purpose of this thesis was to generalize the KKR formalism in KKRnano in such a way that a non-collinear alignment of the atomic spins can be treated. In addition to this, the relativistic coupling of spin and orbital degrees of freedom, which arises from the Dirac equation, was introduced to the code. This coupling gives rise to the Dzyaloshinskii-Moriya interaction (DMI) from which the formation of non-collinear magnetic textures usually originates. Other methodological features that were added to KKRnano or were re-established in the context of this thesis are the Generalized Gradient Approximation (GGA), Lloyd’s formula and a semi-core energy contour integration. GGA is known to be a better approximation to the exchange-correlation energy in DFT than the still very popular Local Density Approximation (LDA), Lloyd’s formula allows to determine the charge density exactly, despite the angular momentum expansion of all quantities, and the semi-core energy contour integration facilitates the treatment of high-lying electronic core states. Furthermore, an experimental port of the multiple-scattering solver routine to Graphics Processing Unit (GPU) architectures is discussed and the large-scale capabilities of KKR nano are demonstrated by benchmark calculations on the supercomputer JUQUEEN that include more than 200.000 atoms. The new version of KKRnano is used to investigate the magnetic B20 compounds B20-MnGe and B20-FeGe as well as alloys of B20-Mn$_{1−x}$Fe$_{x}$Ge type with varied concentration of Mn and Ge. These compounds are well-known for exhibiting helicalstates. Recently reported observations of topologically protected magnetic particles, also known as skyrmions, make them promising candidates for future spintronic devices. Initially, the known pressure-induced transition from a high-spin to a low-spin state in B20-MnGe is reproduced with KKRnano and an examination of the magnetocrystalline anisotropy yields unexpected results. [...]
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